How analog water meters work + trouble-shooting if your water meter isn't working
If you have a 3-in-1 water meter (one without batteries that looks similar to this one below), this guide is for you. As simple as they are, there are still a few basic do's and don'ts that will help you trouble-shoot if you're getting no reading, or low reading even on a recently watered plant...
Don't test water meters in water
They don't work in water. The 2 probes are 2 different types of metal. When you add an electrolyte (that's the moisture in the soil), it works like a weak battery, and reads the 'current' passing between the probes. You need some air space for this to work. Higher moisture = more current, moving the dial more. It’s best never to emerge the probes in a glass of water. Wipe them clean with a damp cloth, don't rinse them under water.
Don't insert water meters close to the pot edge
Next thing to know is don't insert them down the side of your pot. Soil naturally pulls away from the sides as it dries, and water is also naturally 'sucked in and down' towards the middle and bottom of the pot as the substrate dries. A plant that's still moist can read dry if you insert the probes too close to the edge of the pot, or not far enough down. Best to insert them roughly halfway between the edge and the stem, about 3/4 of the way down into the substrate to root level where the water collects.
Don't expect wet readings for indoor plants
Also keep in mind if you get a wet reading when testing an indoor plant something may be wrong. Most water meters are made for indoor and outdoor plants. Unless your indoor plant's soil is very fine and compacted (like you would use outdoors in order to retain water), a recently watered indoor plant should read moist, not wet. Most indoor plants do best in a free draining mix, which should read from dry to moist, but will rarely read wet (unless the probes enter a more compacted, denser area in the pot). A wet reading can indicate your soil is not free draining enough, which for many indoor plants puts them at risk of root rot.
Do insert water meters vertically, not on an angle
Check you insert the probes vertically. Inserting on an angle can mean the 'current' doesn't get picked up by the other probe unless they are lined up.
How to test if a water meter is working
To test if an analog water meter is working, find a plant you know is dry. Put the meter about 3/4 of the way down the pot, and about halfway between the edge and main stem. If you hit an area of resistance, or no resistance at all, you may be in an area that's too dense, may be about to damage a root, or may be in an air pocket. Pick somewhere the meter slides in without having to force it.
Check the reading on the dry plant (take a photo if you like to compare later). Water like normal until water pours out the drainage holes. Test again in the same place. It should now read from moist to wet (note however that it may not read 100% wet, which is a good thing - see above for why).
If you're still worried your meter isn't reading correctly, get some finer or compacted soil that doesn't have a lot of space between the substrate (like your typical potting soil used in outdoor containers), or even some soil from outdoors. Bottom soak it for 15 minutes (or until the surface is saturated and wet to the touch), then test again and you should get a wet reading.
Don't 'store' a water meter in a plant
If you've had your water meter for a while, and usually leave it in a plant between use, the metal probe tips will break down over time (I've seen a difference as fast as a few weeks). Always remove the meter between uses and store wiped and dry.
Do remove the protective caps
This may sound obvious, but only once you know it! If your meter isn't reading, check you've removed the caps off the ends of the probes. Not all meters come with these caps, but they are intended to protect them during transit and storage, so should be removed if yours does have them. If you keep them, you can put them back on after each use as it will protect the metal from degrading over time.
Why a water meter gives different readings from the same plant
It's normal to get a range of readings from the same pot if your potting mix contains more than one substrate, or is compacted in some areas but not others, or you have a very airy, chunky mix. It isn't reading the entire pot, it's reading the area between the probes where you inserted it. If unsure, take at least a couple of readings in different spots to compare.
Don't always water when dry
Remember no water meter tells you when to water. It only tells you the moisture level of the substrate where it matters, down near root level, not at the surface. But you decide when to water. For example, a Peace Lily is usually best watered when the meter says the dry end of moist, and will have wilted by the time it reads dry, but watering a cactus when the reading says moist might cause root rot. It's up to you to decide at what level of moisture your plant should be watered again according to its needs, not what the meter says.
Do top water to flush out excess mineral salts
Potting mix with a high salt content can cause a water meter to give you no reading, or for the dial to swing around. If you have older soil, or you fertilise without top watering, or your tap water is 'hard' and high in minerals, excess mineral salts will build up over time.
A good heavy flush water from the top will help wash away excess salts. Best done at least once a month. If you're noticing white 'crusty' deposits on the surface of your soil, or around the drainage holes, that's also a sign of high mineral salt content in your soil. Other than stopping a water meter from working properly, salt build-up can also stop plants being able to absorb water, and can change the soil pH and burn your plants. If a flush watering isn't enough, you may need to repot into fresh potting mix.
Where can I buy water meters?
From me here at Love That Leaf :) I stock the type of 3 in 1 meters this guide is about, as well as digital water meters, and colour changing water meters. Shop all plant water meters here >